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Six murine macrophage precursor cell lines, thought to be arrested around the CFU-GM stage of the myeloid differentiation and shown to be negative for acid phosphatase, F4/80 antigen expression and phagocytosis capacity, were tested for their ability to differentiate into osteoclasts. Their differentiation potential was compared with that of the haemopoietic stem cell line FDCP-mix C2GM. None of the macrophage precursor cell lines could be induced to differentiate into osteoclasts when the cells were cocultured with either periosteum-free metatarsal bones of fetal mice, or monolayers of osteoblast-like cells. In contrast, when the haemopoietic stem cell line FDCP-mix C2GM, murine fetal liver cells or murine spleen cells were used as a source of haemopoietic precursor cells, numerous osteoclasts were formed in both culture systems. During cell culture a small percentage of the macrophage precursor cells attached to the bottom of the culture well. These firmly attached cells acquired acid phosphatase activity, F4/80 antigen expression and phagocytosis capacity. Furthermore, when the cell lines were cultured for 2 or 4 days with 1% DMSO, up to 30% of the precursor cells differentiated into metamyelocytes. These results suggest that the macrophage precursor cell lines are able to acquire macrophage and granulocyte characteristics, but are unable to differentiate into osteoclasts. In contrast, the haemopoietic stem cell line FDCP-mix C2GM is able to differentiate into both macrophages and osteoclasts. We therefore suggest that the osteoclast lineage branches off at an early stage of the myeloid differentiation pathway.

Recently we reported that the osteoclast originates from the pluripotent hematopoietic stem cell. However, a detailed analysis of the progenitor and precursor stages of the osteoclast lineage is hard to perform with primary cultures of stem cells. In the present investigation interleukin-3 (IL-3)-dependent multipotent hematopoietic stem cell lines (FDCP-mix), which have many characteristics in common with freshly isolated hematopoietic stem cell lines (FDCP-mix), which have many characteristics in common with freshly isolated hematopoietic stem cells, were assayed for their osteoclast formation capacity. FDCP-mix cell lines A4, C2GM, and 15S were cocultured with periosteum-free 17-day-old fetal metatarsal bones. The effects of culture time, medium composition, and addition of WEHI-3b-conditioned medium (an unpurified IL-3 preparation) on osteoclast formation were studied. 15S cells never differentiated into osteoclasts. Both A4 and C2GM cells were able to generate osteoclasts. Osteoclast formation was visualized by staining for tartrate-resistant acid phosphatase activity and confirmed by 45Ca release assays and electron microscopic studies. Medium supplemented with fetal calf serum clearly supported osteoclast formation from A4 cells better than medium supplemented with cock serum. The difference between fetal calf serum and horse serum is generally less pronounced. C2GM cells formed osteoclasts more readily and, generally, earlier than A4 under all culture conditions. WEHI-3b-conditioned medium addition increased the numbers of osteoclasts and their resorption activity. The coculture of stripped metatarsal bones with FDCP-mix cell lines therefore offers a model system with many possibilities for the study of osteoclastogenesis and its regulation.

We investigated the effect of irradiation on the kinetic characteristics of amino acid and glucose transport, and the effect on the activity of the cell membrane-bound enzyme 5'-nucleotidase and on the receptor-mediated stimulation of cyclic adenosine monophosphate synthesis by prostaglandin E1. Irradiation inhibited the sodium-dependent amino acid transport by a reduced binding of the amino acid to the transport unit. The transport of glucose, which appeared to be a sodium-independent process, was temporarily stimulated by increased maximal velocity of the transport. No effect was found on the binding to the transport unit. Irradiation increased the 5'-nucleotidase activity and decreased the prostaglandin E1-stimulated cyclic adenosine monophosphate synthesis 48 h after exposure to 20 Gy. It is concluded that irradiation decreases sodium-dependent transport by impairment of the transport unit, does not impair a sodium-independent process, and has opposite effects on membrane-bound enzyme activity and a receptor-mediated process.

Hemopoietic stem and progenitor cells from different sources differ in radiosensitivity. Recently, we have demonstrated that the multinucleated cell responsible for bone resorption and marrow cavity formation, the osteoclast, is in fact of hemopoietic lineage. In this investigation we have studied the radiosensitivity of osteoclast formation from two different hemopoietic tissues: fetal liver and adult bone marrow. Development of osteoclasts from hemopoietic progenitors was induced by coculture of hemopoietic cell populations with fetal mouse long bones depleted of their own osteoclast precursor pool. During culture, osteoclasts developed from the exogenous cell population and invaded the calcified hypertrophic cartilage of the long bone model, thereby giving rise to the formation of a primitive marrow cavity. To analyze the radiosensitivity of osteoclast formation, either the hemopoietic cells or the bone rudiments were irradiated before coculture. Fetal liver cells were found to be less radiosensitive than bone marrow cells. The D0, Dq values and extrapolation numbers were 1.69 Gy, 5.30 Gy, and 24.40 for fetal liver cells and 1.01 Gy, 1.85 Gy, and 6.02 for bone marrow cells. Irradiation of the (pre)osteoclast-free long bone rudiments instead of the hemopoietic sources resulted in a significant inhibition of osteoclast formation at doses of 4 Gy or more. This indirect effect appeared to be more prominent in the cocultures with fetal than with adult hemopoietic cells. Furthermore, radiation doses of 8.0-10.0 Gy indirectly affected the appearance of other cell types (e.g., granulocytes) in the newly formed but underdeveloped marrow cavity. The results indicate that osteoclast progenitors from different hemopoietic sources exhibit a distinct sensitivity to ionizing irradiation. Radiation injury to long bone rudiments disturbs the osteoclast-forming capacity as well as the hemopoietic microenvironment.

Two cell populations were isolated from calvaria of chick embryos: PF cells were liberated by collagenase treatment from the periosteum, OB cells from the periosteum-free calvarium. Both populations were cultured in plastic culture dishes. After 6 d of culture, monolayers of each cell type either were scraped off the culture dishes, transplanted on the chorio-allantoic membrane of 7-d-old quail eggs, and cultured there for 6 d, or were used for biochemical experiments. OB transplants proved capable of producing calcified bone matrix, whereas PF transplants formed only fibrous tissue. Biochemically, OB cells showed high cAMP production in the presence of parathyroid hormone (PTH), whereas cAMP production was not stimulated in PF cultures. Lactate production was stimulated by PTH in both populations although somewhat differently. Citrate decarboxylation was high in OB cells and was inhibited by PTH but was low in PF cells, where it was stimulated by the same hormone. The differences in hormonal response between the two cell types made it possible to conclude that PF cultures are relatively free of OB cells. The PF contamination in OB cultures was more difficult to assess. The experiments described in this report show that the OB population contains osteoblasts or osteoblastlike cells which are, under favorable circumstances, capable of bone formation.